With increasing sophistication, personal computers interact with users through a variety of multi-media apparatus and techniques. In particular, desktop computers have begun to incorporate signals from a variety of sound sources, including microphones, tape and CD players, video inputs and telephone equipment. These sources provide sound and voice information for use by computer applications. For instance, sampling program applications can capture, store and modify voice and music information. As another example, speech recognition and synthesis programs can parse human speech and perform program commands based upon the recognized human voice. New uses for sound inputs will continue to expand computer performance.
Unfortunately, several factors hamper the use of these sound inputs in desktop computers. The desktop computer, designed for digital purposes, presents a rather hostile environment for audio frequency electronic operation. Desktop computers continuously generate digital signals having spurious electromagnetic frequency components in the audio region, creating common mode noise. In addition, series inductance and series resistance drops (IR drops) within circuit boards, connectors and cables exacerbate this common mode noise. Power mains frequencies and their principal harmonics also add to this common mode noise.
Common mode noise makes connecting two analog devices together (such as a sound input and a desktop computer on-board preamplifier) difficult. The straightforward approach connects the ground signals of the two devices together and then connects the external signal source to a suitable input point. However, because of common mode noise, ground is seldom true "ground". A floating ground, caused by coupled common mode noise, can interfere with and degrade voice and sound signals. Exposed to this interference, program applications can exhibit anomalous and often erroneous behavior. Speech recognition applications, in particular, require fairly high signal-to-noise ratios to extract recognizable words from a stream of sound data. Common mode noise degradation can hamper and even prevent proper operation of these human voice and sound applications, especially from sound sources that provide very low level signals.
Two conventional methods can be tried for connecting audio signals to a desktop computer. One simple method uses a single-ended connection, thereby connecting grounds together and simply hoping that the differences in grounds will not be significant. The other method employs complex and expensive full differential inputs to minimize common mode noise. Unfortunately, the computer industry faces significant competitive pressures to reduce costs and increase reliability and manufacturing yields. Expensive or unreliable techniques for handling common mode noise are simply not viable for most desktop computer applications.
Current apparatus and methods for connecting analog components together do not provide a simple, inexpensive approach for connecting sound inputs to a desktop computer system while minimizing floating ground and noise degradation. These conventional approaches do not provide a high-performance sound input connection and conditioning circuit that is easy to manufacture, with high yields and dependable performance. What is needed is an improved method and apparatus for connecting and conditioning audio signals for computer applications. An improved apparatus and method for connecting and conditioning audio signals should provide a simple, inexpensive and dependable approach for the inputting of sound and human voice signals. The improved apparatus and method should provide a flexible approach that easily adapts to any particular desktop computer environment, and that can be incorporated in computer circuitry with inexpensive, readily available parts.